No need for primer if you're doing a Camo paintjob, when it starts to wear it'll just look better. If you're painting over 2-tone though I'd recommend some good auto primer; it's cheaper and more hard wearing than the krylon stuff.
I buy mine from a local military equipment retailer, not sure on the cheapest place online. Patrol base and landwarriorairsoft both stock it though.
Tips;
Get some stencils if you want to do a camo paintjob, laundry bag mesh, leaves etc are all good.
Use less paint than you think you should.
Wait for each coat to dry before you start the next.
Spray in short bursts from approx 12" away from the gun.
Always start with the spray aiming off the gun and move it on.
Using those above tips you should avoid pooling and crinkly paint, as well as spatter from a semi blocked spray can nozzle :-)